In certain machine tools, such as semiautomatic lathes, center lathes, or drill presses, despite the precision of the machines, it is difficult to perfectly align the axis of the element supporting the boring tool with the axis of the bore to be produced. One of the main causes of these problems is the temperature variations caused by heating, which take the form of uncontrollable expansion phenomena.
To ensure good alignment between the boring tool and the bore, it is known that a floating chuck can be used which automatically and geometrically compensates for errors in alignment and angularity.
In known fashion, the tool-carrying bush is coupled to the chuck by an Oldham coupling and moves on spherical bearings associated with this coupling. Thus, the chuck can shift laterally relative to the body, while maintaining perfect parallelism with the axis of the chuck.
The Oldham coupling comprises a plate which has four openings, shifted angularly pairwise by 90 degrees, in its periphery, said openings serving to accommodate four axial fingers, two of said fingers, 180 degrees apart, being integral with the body, and the other two fingers being integral with the bush, each opening having a depth greater than the diameter of the fingers so as to allow the plate to shift in two crosswise directions. In one of these directions, the bush shifts with the plate relative to the body, while in the other crosswise direction the body shifts with the plate relative to the bush. The plate has openings to accommodate spherical bearings which abut the two parallel surfaces located on both parts of the plate and which are integral respectively with the body and the bush.
This solution is satisfactory in the sense that it allows extremely fine and delicate adjustment of the lateral position of the bush relative to the plate. However, the spherical bearings are only in point contact with the plates, so that the latter must be made of a highly resistant material, and the axial load-carrying capacity of the bush is relatively low because of the excessively high pressures exerted by the spherical bearings on the contact surfaces, resulting in marring of the latter.
The deterioration caused by axial overloading becomes generally evident in the event of abnormal functioning caused by impact, tool breakage, improper operation, or other events.
This axial overload corresponds to an increase in relative pressure between the spherical bearings and the surfaces of the body and the bush between which the bearings roll resulting in destruction of these elements. In such cases, the bush must be replaced.
The spherical bearings and their bearing surfaces have a reversible elastic deformation capability up to a certain degree of deformation. Beyond this value, deformation becomes permanent and the bush is damaged.